The invention concerns a device for the high pressure densification of superconducting wire out of compacted superconductor material or superconductor precursor powder particles, said device comprising four hard metal anvils with a total length parallel to the superconducting wire, said hard metal anvils relied to external independent pressure blocks, which are in turn either fixed or connected to high pressure devices, preferably hydraulic presses.
A device as described above is known from EP 2 172 985 A1.
Ten years after the discovery of superconductivity in MgB2, the fabrication of binary and alloyed MgB2 wires has now reached the industrial level, and kilometer lengths of wires can be produced, which may be used for various applications, e.g. magnetic resonance imaging (MRI), low field NMR magnets, fault current limiters and high current connections. A further application is the partial replacement of the considerably more expensive Nb3Sn wires in high field magnets, particularly in the field range between 9 and 12 T.
A major requirement for the use of MgB2 wires in all mentioned applications is a high critical current density, and important efforts are presently being undertaken for enhancing this quantity. Today the industrial fabrication of MgB2 wires follows two main powder metallurgical routes: the ex situ and the in situ techniques. Both techniques have been described in detail in the review of Collings et al. [1]. The main difference between the ex situ and in situ technique resides in the choice of the precursor powder particles, which are MgB2 or Mg+B, respectively. A third method, the internal Mg fusion process (IMD), by Hur et al. [2], has the potential for large scale production, but has not been applied yet at a large scale.
From the wealth of published data it follows that the main requirements for optimized the critical current densities, Ic, in MgB2 wires are high purity and a submicron size of the constituents B and Mg as well as of the Carbon based additives. In addition, it was found that the Boron nanopowders should be amorphous rather than crystalline. The size of the Boron particles is of great importance, the reaction kinetics leading to final MgB2 grains well below 100 nm [3], thus the most appropriate initial particle size should ideally be in the nanosize range. It is now commonly accepted that the substitution of C on the B lattice sites enhances the residual resistivity ρo and thus the upper critical field, Bc2, as well as the irreversibility field, Birr, of MgB2.
From the presently known data, it follows that the enhancement of ρo within alloyed MgB2 wires, where Carbon partially substitutes Boron in the MgB2 lattice, is the dominant effect enhancing the critical current density, Jc, at high magnetic fields. So far, the pinning behavior of MgB2 has been found to be little affected by the additives. This is confirmed by the relaxation data of Senatore et al. [4], who found for bulk samples with SiC and Carbon additives that the pinning energy Uo of alloyed MgB2 is unchanged with respect to the one of the binary compound. A further confirmation of this statement can be extracted from the data in the work of Collings et al. [1], who analyzed the effect of 42 additives.
A completely different approach for enhancing the transport properties of MgB2 wires consists in enhancing the mass density of the MgB2 filaments. The approach involving densification is particularly adapted to filaments produced by powder metallurgy, which have considerably lower mass densities compared to their theoretical value: in situ MgB2 filaments have been reported to exhibit quite low mass densities, of the order of 45% of the theoretical value, 2.6 g/cm3 [1,5]. Ex situ wires exhibit higher mass densities, but their value does not exceed 70% [1]. Various attempts have been undertaken for enhancing the mass density of bulk MgB2 under high pressure/high temperature conditions. Prikhna et al. [6] reacted bulk alloyed samples at T>1000° C. in a multi-anvil device under pressures 2 GPa, while Yamada et al. [7] performed hot pressing on SiC alloyed in situ tapes at 630° C. under 100 MPa. In the bulk samples [6], the mass density was enhanced to values close to 100%, yielding B(104)=11 T at 4.2K, where B(104) is the field at which Jc reaches a value of 1×104 A/cm2.
In the hot pressed tapes [7], the enhancement of Jc was markedly higher, the extrapolated value of B(104)∥ being close to 14T. However, these tapes have a high aspect ratio, leading to considerably lower values of B(104)⊥, which, however, were not specified. The higher Jc values for hot pressed tapes [7] in comparison with those of hot pressed bulk samples [6] are at least partly due to the fact that the deformation of tapes by rolling leads to a certain degree of texturing. Texturing is an inherent feature of MgB2 wires and tapes produced by multistep deformation. On the basis of MgB2 (002) rocking curves obtained by means of synchrotron X ray diffraction, Hassler et al. [8] have recently reported that the tape rolling procedure creates a texture in Mg which is transferred to the MgB2 crystallites during reaction. Indeed, the reaction to MgB2 starts already at temperatures slightly below 600° C., where Mg is still solid. The directional morphology of the Mg is always visible in cold deformed Mg+B powder mixtures.
As MgB2 wires in kilometer lengths are necessary for industrial applications, this rules out combined high temperature/high pressure processing steps, which can only produce short lengths. Treatments by high isostatic pressure (HIP) can be envisaged, but the subsequent react and wind treatment of already reacted wires for the construction of magnets would be very difficult. It is clear then that the high pressure steps should be applied at low temperature, preferably at room temperature, thus allowing winding and cabling before the reaction heat treatment.
A room temperature processing technique was recently developed at the University of Geneva, namely the Cold High Pressure Densification, or CHPD (see Flükiger et al. [5]). EP 2 172 985 A1 is describing the principle of wire densification. The CHPD method is based on a press/release/travel cycle at room temperature, 4 hard metal anvils transmitting simultaneously a high pressure on all four sides of a square (or rectangular) wire, thus inducing an enhancement of mass density in the MgB2 filaments on a length L. This densification step is followed by a pressure release, thus allowing the forward travel of the wire by a length L1<L, after which the cycle is repeated up to the whole wire length. As reported in [5, 10, 11], the operation with our laboratory device was successful in enhancing the mass density and the value of Jc in MgB2 wires prepared by the in situ technique. A considerable enhancement of Jc was also obtained on ex situ wires.
In in situ wires, the densification step has the effect of enhancing the mass density of the unreacted Mg+B filament. Since the aspect ratio is controlled by the 4 anvils, the degree of texturing remains unchanged [5]. This is in contrast to the effect of pressing a tape between two walls: the tape flows in the direction parallel to the pressing walls, with the consequence that the aspect ratio is strongly enhanced, thus leading to a higher degree of texturing is enhanced, the mass density changing only slightly [6,7].
After applying CHPD on short in situ MgB2 wires, the mass density of binary MgB2 monofilaments after reaction was enhanced from 0.44% to 0.58±0.04% of the theoretical mass density after applying 2.5 GPa [5]. At the same time, a marked decrease of electrical resistance was observed on densified wires, reflecting an improved connectivity. The pressures applied on the 4-wall cell reached a maximum value of 6.5 GPa. At this pressure, the mass density dm of the unreacted Mg+B powder mixture reached 96%, while the corresponding value df in the reacted MgB2 filament increased up to ˜73% of the theoretical value [5]. However, a reproducible enhancement of Jc was only observed up to pressures of the order of ˜3 GPa, higher pressures leading to large scattering Jc, showing no further improvement. The reasons for this limitation are not yet understood, but the behavior of Jc at pressures above 3 GPa is irrelevant for the industrial application of CHPD. Indeed, the practical limit for hard metal anvils submitted to several thousands of pressure steps is of the order of 2 GPa.
The enhancement of Jc in MgB2 wires treated by the CHPD process was observed in monofilamentary and multifilamentary wire configurations, as well for binary and for the alloyed MgB2/Fe wires of [5]. The observed enhancement was higher for the alloyed wires than for the binary ones. In most cases, the wire length was 45 mm, while the pressure length was chosen to L=29 mm. After densification at 1.85 GPa, for binary MgB2 wires at 20K/5T and 4.2K/10T increased by 300% and 53% with respect to the as-drawn wire of the same batch, the reaction conditions being 1 hour at 650° C. After CHPD processing at 2.0 GPa on MgB2 wires with C4H6O5 additions [11] reacted for 1 h at 600° C. an even stronger enhancement of was observed. The value of the magnetic field at which a critical current density of 104 A/cm2 was determined, B(104)∥=1×104A/cm2 at 4.2 K was raised from 11.5T for the original wire to B(104)∥=13.8 T and B(104)⊥=13.2 T, respectively, for fields parallel and perpendicular to the wider face of the rectangular conductor, respectively (at a criterion of 1.0 μV/cm). The corresponding values at 20K were 5.9 and 5.75 T, respectively, while Birr∥ at 20K was −11 T [11]. These values exceed the highest reported critical current densities on in situ MgB2 round wires prepared without pressure: Susner et al. [12] reported for round SiC added MgB2 wires the value of B(104)=12 T, using the 1 μV/cm criterion at 4.2 K.
However, although the pressure cell for CHPD presented in [5, 10] yielded very positive results, this previous device is applicable to short wire lengths only. Indeed, the operation with this device was time consuming: since the pressure release was obtained by loosening a series of screws, the time for a single press/release cycle followed by pressing on a neighbor position of the wire was 10 minutes and more, which is not appropriate for the densification of long wire lengths. In addition, the screws were found to undergo a plastic deformation during the application of pressure, the applied pressure on the elongated intermediate element showing a variation after a certain number of cycles. The uniformity of Jc over the whole wire length being a main requirement for industrial wires, it is imperative that the applied pressure is exactly the same over a very large number of pressing cycles.
Very high pressures applied to the superconducting wires to increase Jc lead to a short lifetime of the hard metal anvils. Exchange of the anvils, however, is not only at the expense of the material, but also interrupts the production cycle for some time.
Thus, the object of the invention is to introduce a device and method which produces high critical current densities Jc at reduced pressure applied to the hard metal anvils.